1. Field of the invention
The present invention concerns the fabrication of an optical fiber preform by the CVD method. It is more particularly concerned with an optical fiber preform fabrication installation including a device for removing excess soot during the fabrication of the preform.
2. Description of the prior art
In a manner well known in the fabrication of an optical fiber preform by the CVD method, silica soot is obtained in a reaction tube from a gaseous mixture injected into the latter and this soot is vitrified by heating it using a mobile torch executing successive passes along the tube rotating on its axis. The soot obtained beyond the travel of the torch is not vitrified. This excess soot is received and deposited in the downstream end part of the reaction tube, beyond the travel of the torch, or in an exhaust tube connected to the reaction tube, substantially all of the length of which is then swept by the torch. It is removed to prevent clogging of the area in which it is deposited and resulting defects in the preform.
Document JP-A 5841734 discloses a device for removing excess soot including a rod mobile in translation and equipped with a terminal screw with flat helical threads which scrape the interior surface of the deposition area to remove the soot into a collecting box.
A device of the above kind is not always satisfactory because the excess soot can accumulate between the threads of the screw so that the latter is rendered ineffective and/or be pushed back towards the preform.
Document WO 89/02 419 describes another device for removing excess soot downstream of a preform during fabrication of the latter. This device includes a screw type structure that rotates in the exhaust tube and is formed by a tubular pipe wound in a spiral. This pipe is a pressurized gas injection pipe and is provided with holes open towards the outlet of the exhaust tube, in order to create a flow of gas entraining the excess soot towards the outlet of the exhaust tube. The tubular screwthreads do not achieve very effective scraping of the deposition area. Moreover the gas pressure in this pipe must be sufficient for the flow of gas in the exhaust tube to be effective. On the other hand, there is the risk of causing turbulence prejudicial to the quality of the preform.
An aim of the present invention is to avoid the drawbacks of the above prior art systems.